Dr. Niven, in his study of 1,021 households previously described, found that 105 families suffered in both the summer and autumn 1918 epidemics. “They comprised a population of 565 persons, of whom 205 suffered in summer and 360 escaped. In the autumn epidemic eighty-two (or 40 per cent.) of the presumably ‘protected’ persons succumbed again, whereas only 120 (or 33 per cent.) of the ‘unprotected’ suffered. Of the former, however, only one died, while five of the latter terminated fatally. These are interesting figures. If they are borne out by subsequent inquiry, they are somewhat difficult of explanation. The persistent susceptibility to the primary disease and yet comparative immunity from the fatal sequel, would seem to suggest a dual infection, against one element of which the body is able to produce protection, while it is unable to do so against the other.” Frost made a canvass of 33,776 individuals in Baltimore between November 20th and December 11th, 1918. The same population was again covered in January, 1919, to determine the extent of the recrudescence reported in December. Among 32,600 people, 724 cases of influenza had occurred in the interval since the first survey. Of this number only 26 or 3.6 per cent. were definitely cases of second attack in the same individual. Even in these cases the diagnosis is necessarily uncertain. Frost says that considering that 23 per cent. of the population had had influenza prior to December 11th, the proportion of second attacks should have been much greater if no immunity had been acquired. A second canvas in San Francisco gave generally corresponding results.